The present invention relates to a method for the recovery of gallium and/or germanium from fly ash. In utility and industrial boilers burning coal, fly ash is produced. Electrostatic precipitators, wet scrubbers, baghouses or other recovery equipment are used to remove the fly ash particulates from the combustion gases. In general, the fly ash is composed of fine grained particles having a silica-alumina base with small amounts of some trace metals concentrated on the surfaces of the particles. Some of these trace metals are valuable, including molybdenum, tungsten, nickel, gallium, and germanium. Others, such as arsenic, lead and antimony, are toxic.
Numerous attempts have been made in the past to recover certain of the trace metals present in fly ash. Report of Investigations 6940 of the United States Department of the Interior, Bureau of Mines, entitled "Extraction of Germanium and Gallium from Coal Fly Ash and Phosphorous Furnace Flue Dust" by R. F. Waters and H. Kenworthy (1967) describes the efforts of the Bureau of Mines to recover germanium and gallium. Sublimation of these trace metals was the method described in the report. Furthermore, U.S. Pat. No. 4,475,993, issued Oct. 9, 1984, describes a process for recovering silver, gallium and other trace metals from a fine grained industrial fly ash. The process involves contacting the fly ash with aluminum trichloride in an alkali halide melt to react the trace metals with the aluminum trichloride to form compositions soluble in the melt and a residue which contains the silicate and aluminum oxide. Then, the desired trace metals are separated from the melt by electrolysis or other separation techniques.
It is an object of this invention to provide a method for the recovery of gallium and/or germanium from fly ash which is better than and more economical than the methods described above. The use of the oxidizing atmosphere step is an improvement over the Bureau of Mines' method in that arsenic and sulphur are removed from the fly ash so that they will not remain in the concentrated gallium and germanium fraction which is recovered.